How to change the inherited F-curve independently from the original one ?
You could also try to use F-Curve-Modifiers. To do so:


Go to frame 0 and add a keyframe on the rotation Axes you want
In the Graph Editor bring up that sidebar by pressing N. Add a Modifier > Generator. This will give you the polinominal tools to create a Graph as in y=mx+b as you might remember from school.


This value controls the ascent of the curve. By default it's 1, so we have y=0+1x, so y=x meaning that for each 1 frame (one unit on the x axis), the curve ascends 1 unit on the y axis as well (it actually goes up to 57,296 because 1 rad= 57,296°). 




So if you want to have a second object that rotates half of that speed, you just enter half of the value you entered at (3), 0.5 in my example.